I am surprised at some of the remarks made on this thread.
I teach Judo, Jiu-jitsu, and Aikido.
I totally disregard all of the Asian philosophical 'meditative' non-sense that can be found in all three (Brazilain Jiu-jitsu does not generally have this problem as does traditional Japanese jiu-jitsu).
Now, since I have been teaching, I have had two challenge matches with black belts in Aikido, and one challenge match with an expert in Hap-ki-do.
The first challenge match I had with a black belt in Aikido was a quick match. Clinch, takedown, mount, back-mount, and finished him with hadaka-jime (mate-leon--or the sleeper). No punches were thrown. He really tried his game. Problem was, I know the game too. I was not going to let him exagerate my momentum and break my balance. Once I had the clinch, he was outside the realm of Aikido. I was impressed with his Aikido technique, but he was more interested in challenging me. I encouraged him to return, just to practice Aikido with me if nothing else (he was good at it), but he never came back.
The second challenge match that I had with an Aikido black belt happened last year. He teaches the system in a small neighboring town near my own. He came into my school, demanded to see my Aikido technique. I was teaching a jiu-jitsu class, but since there were very few students there, I agreed to demonstrate some of my Aikido. I demonstrated the unbendable arm and ko-te-gaeshi. Then this guy comes on to my tatami mats with his shoes and starts contradicting me. Well, I explained to him that if all he knew was Aikido, and he was not versed in Judo or Jiu-jitsu, then he would not have a grappling foundation to fall back on should his Aikido fail (guys, most teachers of Aikido that I know are also black belts in Judo--and for a good reason). He did not like the sound of that. He insisted that he could handle me. No problem, I didn't like his attitude, so I challenged him right there. After I got the clinch, he tried a half-ass version of O-soto-gari, and I laid him down like a baby. I took the mount (tate-shiho-gatame) and pinned him. He struggled a bit (even tried to claw my eye), then asked me to let him up. I said, "No, you have to get up yourself. Use your Aikido."
After he gave up again I let him up. I won the match without applying a choke or joint-lock. Niether Aikido practitioner was hurt during my challenge matches with them. And this is why I like Judo and Jiu-jitsu (as well as Aikido), you can often defend yourself fully, and without suffering injury, and without injuring your opponent.
Although Hapkido is not exactly the same as Aikido, my challenge match with the Hapkido expert was even quicker. I armbarred the guy in 7 seconds. He was trained by one of my first teachers. A Korean master at Hapkido and Yudo (that is how the Koreans say Judo).
I have used Aikido in real fights. It will work if the attacker is committed to his attack with great force, and if you can recognize his attack in time to pull off the Aikido tactic that best suits his attack. This is easier said than done. It is safer just to close the distance and establish a clinch.
I have even used Aikido in wrestling matches with Jiu-jitsu players. I caught a guy pushing really hard ( he clearly advertised it) and he ended up on his face. I have only pulled this off once though. It was funny. He gave up after going face down and started laughing himself. The last time I tried to capitalize on a jiu-jitsu player pushing with an Aikido tactic ended up with me being slung across the floor ( I was a bit late).
Remember, Aikido will work, but it has to be under specific conditions. If those conditions are negated, then so is Aikido. That is why I recommend learning Judo and Jiu-jitsu first, then Aikido. Aikido is like a rifle. You can use it to take care of an adversary at a long range. Judo is like 357 magnum and Jiu-jitsu is like a sawed off shot-gun (you don't want to be in front of that gun when it goes off).
So, study them all, but know your limitations.
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